Color mixing is essential to achieving white light illumination for a variety of applications ranging from street and automotive lighting, to residential, LCD displays and entertainment lighting. White light is typically achieved by illuminating phosphor materials with blue light that emit red and green light. When the three colors combine, the overall emitted light looks white. Such an approach is depicted in FIG. 1, where blue light 101 emitted from a p-n junction illuminates phosphors 102 that emit red 103 and green 104 light. Combination of the three colors produces white light.
The foregoing approach for combining light of distinct colors suffers from three main disadvantages:                The phosphors tend to degrade over time. This adversely affects the color output lifetime as well as the overall efficiency of the device.        The overall device has lower efficiency than normal 3-color LED systems. This is primarily due to heat loss from Stokes shift (inelastic scattering) and phosphor-related degradation problems. Hence, the high intrinsic efficiency of the LED is not achieved.        The system suffers from poor color stability and rendering. Color control for high-quality lighting is difficult with this approach since it is almost impossible to control the relative light output at the different colors.        
Another approach to the generation of white light employs three separate LEDs at the primary colors, red, green and blue (RGB). This allows for independent control of intensity at each color, hence much better color gamut in the output light. In the simplest case, the three LEDs are simply packaged in a single, hermetically sealed, case. As no method for high efficiency three-color mixing has been proposed, the three colors mix in the course of propagation and white light is produced. This produces significant non-uniformities in the white light output and it is difficult to control the quality of the white light. Color mixing methods that involve the use of diffusers suffer from intrinsic efficiency losses due to absorption and scattering of the light in the diffuser.